Binding of high molecular weight kininogen to human endothelial cells is mediated via a site within domains 2+3 of the urokinase receptor

The urokinase receptor (uPAR) binds urokinase-type plasminogen activator (uPA) through specific interactions with uPAR domain 1, and vitronectin through interactions with a site within uPAR domains 2 and 3. These promote the expression of cell surface plasminogen activator activity and cellular adhesion to vitronectin, respectively. High molecular weight kininogen (HK) also stimulates the expression of cell surface plasminogen activator activity through its ability to serve as an acquired receptor for prekallikrein, which, following its activation to two1 chain HK (HKa), may directly activate prourokinase. Thus, interactions of Hka with uPAR might be of importance in regulating both cell surface plasminogen activator activity as well as cell adhesion. To investigate this issue, we determined whether the binding of HKa to human umbilical vein endothelial cells (HUVEC) involved uPAR. HKa bound specifically and sarurably to HUVEC, in a zinc-dependent manner. Binding was potently inhibited by vitronectin, suggesting the interaction of HKa with either uPAR or a vitronectin receptor, such as integrin a. However, HKa binding was not inhibited by fibrinogen, an ab3 ligand, or mAb 7E3, an inhibitory antibody which recognizes the bj integrin chain. Binding was inhibited by anti-uPAR domain 2+3 antibodies, as well as soluble uPAR (uPAR 1-281). Furthermore, suPAR and HKa formed a specific, zinc-dependent complex, as demonstrated using gel filtration. These studies demonstrate that the uPAR is a receptor for HKa on HUVEC, and suggest that interactions of HKa with endothelial cell uPAR may underlie its ability to promote kallikrein-dependent cell surface plasmin generation, and also explain, in part, its anti-adhesive properties.